This invention relates to corona charging arrangements and, more particularly, to a DC biased AC corona charging arrangements.
The use of a corona discharge device has been conventional in xerographic copiers since the inception of commercial xerography. A corona discharge device, or xe2x80x9ccoronodexe2x80x9d, can be a fine wire or an array of points which ionizes air molecules when a high voltage is applied. Originally, a DC voltage of 6 to 7 thousand volts was applied to a coronode in xerographic copiers to ionize the adjacent air molecules causing electric charges to be repelled from the coronode and attracted to an adjacent lower potential surface such as that of the photoreceptor to be charged. In the absence of control, however, such charging arrangements tend to deposit excessive and nonuniform charges on the adjacent surface.
In order to control the application of charges to the adjacent surface so as to provide a uniform charge distribution and avoid overcharging, a conductive screen has been interposed between a coronode and the surface to be charged. Such screened corona discharge devices are referred to as xe2x80x9cscorotronsxe2x80x9d. Typical scorotron arrangements are described in the Walkup U.S. Pat. No. 2,777,957 and the Mayo U.S. Pat. No. 2,778,946. Early scorotrons, however, reduced the charging efficiency of the corona device to only about 3%. That is, only about three out of every one hundred ions generated at the coronode reached the surface to be charged. They also exhibited poor charging uniformity control sometimes allowing the surface to be charged to a voltage exceeding the screen potential by 100% or more. Improved scorotrons now in use usually control surface potentials to within about 3% of the reference voltage applied to the screen and operate at efficiencies of about 30% to 50% but they tend to be complex and correspondingly expensive. The Mott U.S. Pat. No. 3,076,092 discloses a DC biased AC corona charging arrangement which does not require a control screen.
Because such corona charging devices ionize the oxygen and nitrogen molecules in the air, they usually generate ozone to an undesirable extent as well as nitrate compounds which tend to cause chemical corrosion. Usually, large charging devices are required to provide a high current capability because of a tendency to produce arcing between the coronode and low voltage conductors of the charging device or the surface being charged at high charging rates.
Another corona charging arrangement contains a row, or two staggered rows, of pins to which a high voltage is applied to produce corona generating fields at the tips of the pins.
Still another corona charging arrangement, called the xe2x80x9cdicorotronxe2x80x9d, has a glass coated corona wire to which an AC voltage is applied and an adjacent DC electrode which drives charges of one polarity charge toward the photoreceptor to be charged while attracting the opposite polarity charges to itself. Dicorotrons, however, are fragile and expensive and, because of the much larger coated wire radius, require very high AC voltages (8-10 kV). They also generate high levels of ozone and nitrates and require substantial spacing of the corona wire from low voltage conducting elements and the surface to be charged in order to avoid arcing.
Accordingly, it is an object of the present invention to provide a corona charging arrangement having improved efficiency and increased cost effectiveness compared to conventional charging arrangements.
Another object of the invention is to provide a corona charging arrangement by which ozone generation and nitrate production and resulting chemical corrosion are reduced while permitting higher levels of charging current so as to provide high charging rates without arcing.
Still another object of the invention is to provide a corona charging arrangement in which the coronode itself is the only conductive member which determines the equilibrium potential to which the charge-receiving surface is to be charged.
An additional object of the invention is to provide a compact charging arrangement capable of charging a surface at high rates without arcing.
These and other objects of the invention are attained according to one aspect of the invention by providing a corona charging arrangement having a coronode supplied with an AC potential and a DC bias and an insulating structure adjacent to or shielding the coronode. Applying a DC bias to a high frequency AC corona voltage causes the adjacent insulating structure or shielding for the coronode to be charged to a voltage corresponding to the DC bias, and the surface to be charged, such as a photoreceptor, tends to approach the same DC potential as the adjacent insulating structure which is also exposed to the corona generated ions, providing a consistent, dependable and efficient corona charging arrangement.
In one embodiment, a coronode is affixed to and supported by a screen mesh made of insulating fibers and extending parallel to the surface to be charged and in another embodiment a plurality of insulating filaments held by one or more insulating supports embrace the coronode. Parallel insulating filaments which extend between spaced insulating support members and pass on opposite sides of a coronode may be used to support the a coronode which is in the form of a corona wire. In addition, an insulating dielectric member to which a corona wire is held by electrostatic attraction may be used to support the coronode to avoid shadowing or eclipsing of corona generated charges by any part of the support structure. In another embodiment an insulating coating is provided on a corona wire and a capacitor is connected between the AC source and the coronode. The corona charging arrangement of the invention may be used, for example, to provide uniform charge on the surface of the photoreceptor prior to image exposure or for effecting transfer of a toner image from a photoreceptor to a substrate such as paper, or in any other application in which conventional corona charging arrangements are used.
In another embodiment of the invention an insulating structure protects the coronode but makes no contact with it and is arranged so that the capacitance of the coronode to the surface to be charged is greater than the capacitance of the coronode to the adjacent insulating structure. In this embodiment, the only conductive source of the DC bias which determines the asymptotic final potential to which the charge-receiving surface is charged is the DC biased AC coronode itself.